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Isomerases 237
(3) Suggest a mechanism for the hydrolase enzyme below, which is involved in
an aromatic degradation pathway in Escherichia coli.
CO 2
−
O
+ H 2 O
hydrolase
+
CO 2
−
− O 2 C
OH
CO 2
−
− O 2 C
OH
(4) Peptidyl–proline amide bonds are sometimes found in a kinetically stable
cis-conformation rather than the thermodynamically more favourable
trans-conformation. An enzyme activity has been found that is capable of
catalysing the cis–trans isomerisation of such peptidyl–proline amide
bonds, as shown below (Note: This enzyme is strongly inhibited by the
immunosuppressant cyclosporin A shown in Figure 1.3.) Suggest possible
mechanisms for this isomerase reaction.
O
N
peptidyl-proline
cis-trans isomerase
O
O
N
O
(5) Suggest step-wise and concerted mechanisms for the phosphoenol pyruvate
(PEP) mutase reaction shown below. How might you distinguish between
these mechanisms
− O 2 C
PEP
mutase
− O 2 C
2− O 3 PO O PO 3
2−